An optical fiber propagates light by total internal reflection. A light beam introduced into one end of the optical fiber is propagated essentially without any loss of energy to the other end of the optical fiber, even though the optical fiber may be quite long. This property of the optical fiber is utilized in light-based communication systems in which information is encoded onto a light beam, and the light beam is introduced into the optical fiber, propagated through the optical fiber, received at the other end of the optical fiber, and processed to recover the information. Because of its high frequency, the light beam may carry a great deal of information.
The capacity of the optical fiber to carry information may be further increased by simultaneously transmitting a number of light beams through the optical fiber. For example, a first-band light signal of a first wavelength (i.e., a first channel) may carry information for a first user, a second-band light signal of a second wavelength (i.e., a second channel) may carry information for a second user, and so on. The single optical fiber may carry a large number of such light signals, each in a slightly different wavelength band.
One of the practical problems of such an optical fiber light transmission system carrying many light signals in different wavelength bands is that the light signals must be added to or dropped from the main light beam as needed. That is, at a single location it may be necessary to add or drop the first-band light signal for a first period of time, and thereafter to add or drop the second-band light signal for a second period of time. Techniques are known for selectively performing the add/drop function, using tunable etalons, gratings, or graded thin-film coatings. However, these approaches are not fully satisfactory in that they are cumbersome, have high insertion loss, and have limited selectivity and channel isolation for wavelength bands.
There is therefore a need for an improved approach to the add/drop function in such optical fiber light transmission systems. The present invention fulfills this need, and further provides related advantages.